Let’s consider a virus like Human Immunodeficiency Virus (HIV). If we want to study it in a lab setting, we need an animal model, that is a living entity that we can infect and observe for a certain period of time. We cannot infect humans for obvious reasons. Monkeys and chimpanzees are expensive, and research on them subject to increasingly strict ethical guidelines. And the virus cannot infect normal mice. So what to do? The answer may lie in “humanized mice”.

 What are humanized mice?

Humanized mice are otherwise ordinary breeds of lab mice that have human cells, tissues, or organs. These types of mice are frequently used by immunologists, who give the mice a mostly-human immune system, but they can also be used to study other body systems. This permits researchers to study different aspects of human biology and the effects of certain drugs or disease treatments using experiments that would otherwise be unethical to conduct on a human being and don’t have safer substitutes.

How can humanized mice help us?

Humanized mice that have human-like livers are a good model to study potential side effects of drugs on the human liver, a major concern during drug development. Recently, researchers have used humanized mice to retrospectively analyze an experimental hepatitis B drug that led to the death of five patients in a small clinical trial. Preclinical experiments in ordinary lab animals failed to show that the drug was toxic and it was thus thought to be safe. However, subsequent tests on humanized mice revealed the same dangers that the human patients had faced: the mice experienced acute liver failure, and in some instances, death. This alerted scientists to the notion that humanized mice are a better model for toxicological studies of drug candidates than ordinary mice. Had they been used from the start, the dangerous drugs wouldn’t have received human trial approval and those human deaths would have been prevented [], [].

Humanized mice have also been used in other studies relevant to human health. When humanized mice are vaccinated, they can elicit an antibody immune response at levels comparable to that of humans []. Hence, they have been used to probe the development and function of our immune system. Furthermore, they have been used to study hematopoiesis, the process of formation of the human blood’s cellular components, which is difficult to study in humans for ethical reasons.

In addition, humanized mice present a particularly good animal model to study viruses that cannot infect mice, such as HIV []. HIV requires certain molecules to “sneak into” human cells, and these molecules are not found on mouse cells. However, because the cells of humanized mice come from humans and do have these molecules, HIV can establish a stable infection within humanized mice. Therefore, HIV-infected humanized mice can be used to test potential new HIV drugs or to study the human immune response to HIV vaccines, cutting down on the need for primate research.

How are mice humanized?

The process of humanizing mice requires the transfer of 17-19 week old human fetal tissue into young mice, 6-8 weeks of age, with weakened immune systems. If a mouse with a normal immune system received human tissue, it would start to reject the tissue immediately after the transfer because its immune system would recognize the added tissue as foreign. This process, graft rejection, also happens in humans who receive incompatible organ transplants. Therefore, researchers use mice with a genetically weakened immune system for the humanization procedure. These genetically immunosuppressed mice tolerate the presence of foreign human tissue for long enough to conduct experiments [].

Because a mouse’s body is full of its own immune cells, it is essential to remove these cells to create “space” for the human immune cells to populate. To do this, the mice are subjected to a sub-lethal dose of irradiation. Next, to create a niche where the cells can be constantly replenished, the scientists make a small surgical incision to implant human fetal liver and thymus tissue underneath the mouse’s kidney. Finally, human stem cells are injected directly into the mouse’s blood. 17-20 weeks after this procedure, the reconstitution is considered complete when the cells have divided sufficiently to populate their “home” in the mouse’s body. A number of variations to the technique can be introduced depending on the experimental question that needs to be addressed, but most labs follow this general procedure, which is depicted in figure 1.

Figure 1 ~ Humanizing mice. Mice are first irradiated to make room for human hematopoetic cells, cells from the fetal liver and from the fetal thymus. The cells are then able to populate the mouse which can used for further study later. Image adapted from Slides of Dr. Thorsten Mempel, Adamopoulou et al., 2013.

Why are humanized mice still imperfect?

Although humanized mice can now be generated with relative ease, there are still ways in which they could be made more useful.  For instance, the life span of these mice is shorter than that of their ‘normal’ counterparts. Eventually, they will develop a condition known as Graft versus Host Disease (GvHD) where they will start rejecting the implanted human cells. This forces researchers to conduct their experiments in the window between reconstitution and the onset of GvHD.

Although specific genetic lines of lab mice are carefully bred to be very similar, individuals are still subject to inherent biological variation. Mice born on the same day from the same parents respond differently to grafts from the same tissue. Therefore, researchers have to check levels of human immune cells within every mouse to make sure that the process was successful.

Where do we go from here?

Humanized mice are very useful, yet still cumbersome experimental tools. Recently a team in Germany was able to improve the humanization process by eliminating the irradiation step []. However, the narrow window during which scientists must perform their experiments—after humanization but before GvHD takes hold—is still a major obstacle. However, it is possible that in the near future, scientists may establish a strain of genetically humanized mice.

To see why genetic changes might be beneficial, consider seedless watermelons.  To obtain them, we could either plant normal watermelons and remove the seeds from them at maturity, or we could start from the beginning by planting seedless watermelon plants. What I am proposing is a similar idea. Instead of replacing the mouse immune system with that of the human after the mice have been born, we could potentially have a strain of mice in which genes for human immune components could replace those of mouse immune components, allowing the mice to produce human immune organs from birth and eliminating the need for tissue grafting. This improvement will make humanization easier and faster, facilitating large-scale testing on treatments for diseases, like HIV, that previously could not be studied in mice. Hopefully humanized mice can thus help us to overcome hurdles that have prevented us from curing such diseases.

Radwa R. Sharaf is a graduate student at HMS in Thorsten Mempel’s lab. She studies how HIV exploits the immune system to its own advantage

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